For use in a type of internal combustion engine in which fuel is injected directly into a cylinder, there has been known a fuel injection control system equipped with a low pressure fuel pump for sucking fuel from a fuel tank and a high pressure fuel pump for boosting the pressure of the fuel sucked by the low pressure pump to a pressure that allows injection into the cylinder.
In the above-described fuel injection control system, it is desired in order to reduce energy consumption in the operation of the low pressure fuel pump that the discharge pressure (or feed pressure) of the low pressure fuel pump be made as low as possible. However, if the pressure in a section between the low pressure fuel pump and the high pressure fuel pump becomes lower than the saturation vapor pressure of the fuel, vapor might be generated in the high pressure fuel pump.
As a countermeasure against this, Patent Document 1 describes a technology in which when the duty cycle of the high pressure fuel pump becomes equal to or larger than a predetermined value, the feed pressure is raised on the assumption that vapor is generated.
Patent Document 2 discloses a technology applied to a system in which the rate of change in the fuel pressure in a fuel pipe is obtained and a presumption of the generation of fuel vapor is made based on the rate of change thus obtained. In this system, the target fuel pressure is increased when it is presumed that vapor is generated, and the target fuel pressure is decreased when it is presumed that vapor is not generated.
Patent Document 3 discloses a technology in which whether or not fuel vapor will be generated while the engine is shut down is predicted based on the ambient air temperature and the alcohol concentration in the fuel, and when the generation of vapor is predicted, the fuel pressure is raised upon shutting down the engine.
Patent Document 4 discloses a technology in which it is determined whether or not vapor is likely to be generated based on the concentration of vaporized fuel in the gas supplied to an internal combustion engine by a vaporized fuel processing apparatus, and if it is determined that vapor is likely to be generated, the discharge flow rate of a fuel pump is increased.